Init pipeline in the function

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Init pipeline in the function

Igor Mironchick
Hi.

I'm a new with gstreamer and glib.

Why can't I init pipeline in function and then use it somewhere else.

Here is the code example:

void initPipeline( GstElement * pipeline )
{
  pipeline = gst_pipeline_new( "camera" )
}

void main()
{
  GstElement * pipeline;

  initPipeline( pipeline );

  // And here pipeline == 0 !!! Why?
}

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Igor Mironchick

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Re: Init pipeline in the function

Tristan Matthews-2
You're passing the pointer by value, you need to pass its address. Also, you need to call gst_init beforehand, so your code should look like:

#include <gst/gst.h>

void initPipeline(GstElement **pipeline)
{
    *pipeline = gst_pipeline_new("camera");
}

int main(int argc, char **argv)
{
    gst_init(&argc, &argv);
    GstElement *pipeline;
    initPipeline(&pipeline);
    return 0;
}

2010/4/22 Igor Mironchick <[hidden email]>
Hi.

I'm a new with gstreamer and glib.

Why can't I init pipeline in function and then use it somewhere else.

Here is the code example:

void initPipeline( GstElement * pipeline )
{
  pipeline = gst_pipeline_new( "camera" )
}

void main()
{
  GstElement * pipeline;

  initPipeline( pipeline );

  // And here pipeline == 0 !!! Why?
}

--
Regards,
Igor Mironchick

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Tristan Matthews
email: [hidden email]
web: http://tristanswork.blogspot.com

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Re: Init pipeline in the function

wl2776
Administrator
In reply to this post by Igor Mironchick
You can't change the function arguments in C, as they are passed by value.
You can, however, pass a pointer (a memory address) to a function, and it can write something to that address. This will cause changing of values of some variables outside the function.

Igor Mironchick wrote
Why can't I init pipeline in function and then use it somewhere else?
You must return the pointer from the function.

Igor Mironchick wrote
Here is the code example:

void initPipeline( GstElement * pipeline )
{
  pipeline = gst_pipeline_new( "camera" )
}

void main()
{
  GstElement * pipeline;

  initPipeline( pipeline );

  // And here pipeline == 0 !!! Why?
}
gst_pipeline_new allocates the memory and stores it in the variable pipeline, but this variable is lost when the function returns.
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Re: Init pipeline in the function

wl2776
Administrator
wl2776 wrote
You can't change the function arguments in C, as they are passed by value.
I mean, make outside-visible changes of arguments.
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Re: Init pipeline in the function

Igor Mironchick
In reply to this post by Tristan Matthews-2


You're passing the pointer by value, you need to pass its address. Also, you need to call gst_init beforehand, so your code should look like:

#include <gst/gst.h>

void initPipeline(GstElement **pipeline)
{
    *pipeline = gst_pipeline_new("camera");
}

int main(int argc, char **argv)
{
    gst_init(&argc, &argv);
    GstElement *pipeline;
    initPipeline(&pipeline);
    return 0;
}

Thx for your reply. I've already found the solution. But a little bit in another way:

void initPipeline( GstElement *& pipeline )
{
  pipeline = ...
}

void main()
{
  ...
  GstElement * pipeline;
  initPipeline( pipeline );
  ...
}



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Igor Mironchick

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Re: Init pipeline in the function

giorgio-2
Try in this Way:

#include <gst/gst.h>

GstElement* initPipeline()
{
    return(gst_pipeline_new("camera"));
}

int main(int argc, char **argv)
{
    gst_init(&argc, &argv);
   
    GstElement *pipeline = initPipeline();

    //
    // Now you can use pipeline  
    //

    return 0;
}


Giorgio

Il giorno gio, 22/04/2010 alle 10.58 +0300, Igor Mironchick ha scritto:

>
>
>         You're passing the pointer by value, you need to pass its
>         address. Also, you need to call gst_init beforehand, so your
>         code should look like:
>        
>         #include <gst/gst.h>
>        
>         void initPipeline(GstElement **pipeline)
>         {
>             *pipeline = gst_pipeline_new("camera");
>         }
>        
>         int main(int argc, char **argv)
>         {
>             gst_init(&argc, &argv);
>             GstElement *pipeline;
>             initPipeline(&pipeline);
>             return 0;
>         }
>
> Thx for your reply. I've already found the solution. But a little bit
> in another way:
>
> void initPipeline( GstElement *& pipeline )
> {
>   pipeline = ...
> }
>
> void main()
> {
>   ...
>   GstElement * pipeline;
>   initPipeline( pipeline );
>   ...
> }
>
>
>
>
> --
> Regards,
> Igor Mironchick
> ------------------------------------------------------------------------------
> _______________________________________________
> gstreamer-devel mailing list
> [hidden email]
> https://lists.sourceforge.net/lists/listinfo/gstreamer-devel



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Re: Init pipeline in the function

Kulecz, Walter (JSC-SK)[WYLE INTEG. SCI. & ENG.]
In reply to this post by Igor Mironchick
uisng:

(* pipeline) = gst_pipeline_new( "camera" );

in initPipeline should work, but I don't see what this extra function call in main() actually buys you.
________________________________________
From: Igor Mironchick [[hidden email]]
Sent: Thursday, April 22, 2010 2:08 AM
To: Discussion of the development of GStreamer
Subject: [gst-devel] Init pipeline in the function

Hi.

I'm a new with gstreamer and glib.

Why can't I init pipeline in function and then use it somewhere else.

Here is the code example:

void initPipeline( GstElement * pipeline )
{
  pipeline = gst_pipeline_new( "camera" )
}

void main()
{
  GstElement * pipeline;

  initPipeline( pipeline );

  // And here pipeline == 0 !!! Why?
}

--
Regards,
Igor Mironchick

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Re: Init pipeline in the function

Edward Hervey
In reply to this post by Igor Mironchick
The correct answer since nobody was able to come up with it (although
one was very close):

--------------------------------
static GstElement * createPipeline (void) {
  GstElement * pipeline;

  pipeline = gst_pipeline_new ("mylittleponypipeline");

  /* Do stuff with the pipeline, fill it in, etc... */

  return pipeline;
}

void main (int argc, gchar **argv) {
  GstElement * pipeline;

  /* Initialize GStreamer, parse arguments, etc.. */

  pipeline = createPipeline ();

  /* Use pipeline */
}
---------------------------------------

  I recommend everybody who replied to the list to read Kernighan &
Ritchie "The C programming language" [1] as well as reading examples and
learning how to use google.

   Edward

[1] http://en.wikipedia.org/wiki/The_C_Programming_Language_%28book%29

On Thu, 2010-04-22 at 10:08 +0300, Igor Mironchick wrote:

> Hi.
>
> I'm a new with gstreamer and glib.
>
> Why can't I init pipeline in function and then use it somewhere else.
>
> Here is the code example:
>
> void initPipeline( GstElement * pipeline )
> {
>   pipeline = gst_pipeline_new( "camera" )
> }
>
> void main()
> {
>   GstElement * pipeline;
>
>   initPipeline( pipeline );
>
>   // And here pipeline == 0 !!! Why?
> }
>
> --
> Regards,
> Igor Mironchick
> ------------------------------------------------------------------------------
> _______________________________________________
> gstreamer-devel mailing list
> [hidden email]
> https://lists.sourceforge.net/lists/listinfo/gstreamer-devel



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Re: Init pipeline in the function

Tristan Matthews-2
2010/4/23 Edward Hervey <[hidden email]>
The correct answer since nobody was able to come up with it (although
one was very close):


Could you clarify what you considered incorrect in my response?
 
--------------------------------
static GstElement * createPipeline (void) {
 GstElement * pipeline;

 pipeline = gst_pipeline_new ("mylittleponypipeline");

 /* Do stuff with the pipeline, fill it in, etc... */

 return pipeline;
}

void main (int argc, gchar **argv) {
 GstElement * pipeline;

 /* Initialize GStreamer, parse arguments, etc.. */

 pipeline = createPipeline ();

 /* Use pipeline */
}
---------------------------------------

 I recommend everybody who replied to the list to read Kernighan &
Ritchie "The C programming language" [1] as well as reading examples and
learning how to use google.

  Edward

[1] http://en.wikipedia.org/wiki/The_C_Programming_Language_%28book%29

On Thu, 2010-04-22 at 10:08 +0300, Igor Mironchick wrote:
> Hi.
>
> I'm a new with gstreamer and glib.
>
> Why can't I init pipeline in function and then use it somewhere else.
>
> Here is the code example:
>
> void initPipeline( GstElement * pipeline )
> {
>   pipeline = gst_pipeline_new( "camera" )
> }
>
> void main()
> {
>   GstElement * pipeline;
>
>   initPipeline( pipeline );
>
>   // And here pipeline == 0 !!! Why?
> }
>
> --
> Regards,
> Igor Mironchick
> ------------------------------------------------------------------------------
> _______________________________________________
> gstreamer-devel mailing list
> [hidden email]
> https://lists.sourceforge.net/lists/listinfo/gstreamer-devel



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Tristan Matthews
email: [hidden email]
web: http://tristanswork.blogspot.com

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Re: Init pipeline in the function

Tristan Matthews-2
2010/4/23 Tristan Matthews <[hidden email]>
2010/4/23 Edward Hervey <[hidden email]>

The correct answer since nobody was able to come up with it (although
one was very close):


Could you clarify what you considered incorrect in my response?

Unless it's a mixed declarations and code issue (C90 and earlier), in which case you'll have to forgive my c++ contaminated style :)
 
 
--------------------------------
static GstElement * createPipeline (void) {
 GstElement * pipeline;

 pipeline = gst_pipeline_new ("mylittleponypipeline");

 /* Do stuff with the pipeline, fill it in, etc... */

 return pipeline;
}

void main (int argc, gchar **argv) {
 GstElement * pipeline;

 /* Initialize GStreamer, parse arguments, etc.. */

 pipeline = createPipeline ();

 /* Use pipeline */
}
---------------------------------------

 I recommend everybody who replied to the list to read Kernighan &
Ritchie "The C programming language" [1] as well as reading examples and
learning how to use google.

  Edward

[1] http://en.wikipedia.org/wiki/The_C_Programming_Language_%28book%29

On Thu, 2010-04-22 at 10:08 +0300, Igor Mironchick wrote:
> Hi.
>
> I'm a new with gstreamer and glib.
>
> Why can't I init pipeline in function and then use it somewhere else.
>
> Here is the code example:
>
> void initPipeline( GstElement * pipeline )
> {
>   pipeline = gst_pipeline_new( "camera" )
> }
>
> void main()
> {
>   GstElement * pipeline;
>
>   initPipeline( pipeline );
>
>   // And here pipeline == 0 !!! Why?
> }
>
> --
> Regards,
> Igor Mironchick
> ------------------------------------------------------------------------------
> _______________________________________________
> gstreamer-devel mailing list
> [hidden email]
> https://lists.sourceforge.net/lists/listinfo/gstreamer-devel



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web: http://tristanswork.blogspot.com



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email: [hidden email]
web: http://tristanswork.blogspot.com

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Re: Init pipeline in the function

Edward Hervey
In reply to this post by Tristan Matthews-2
On Fri, 2010-04-23 at 10:45 -0400, Tristan Matthews wrote:
>
> Could you clarify what you considered incorrect in my response?

  You answer was compilable code alright.
  Just way too obfuscated for a simple answer which, considering the
basic error the person asking the question did, was much needed :)

   Edward


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Re: Init pipeline in the function

Tristan Matthews-2

2010/4/23 Edward Hervey <[hidden email]>
On Fri, 2010-04-23 at 10:45 -0400, Tristan Matthews wrote:
>
> Could you clarify what you considered incorrect in my response?

 You answer was compilable code alright.
 Just way too obfuscated for a simple answer which, considering the
basic error the person asking the question did, was much needed :)

Fair enough, I wanted to demonstrate pointer semantics...but a stack overflow link might have been more appropriate.

Best,
Tristan
 

  Edward


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